Polymeric foams can be generally classified as either closed-cell foams or as open-cell foams. Open-cell foams can be used as a matrix to contain various liquids and gases. They are capable of various industrial applications such as, for example, use in wipes and diapers, as carriers and ion exchange resins. For some of these applications, it is desirable to have porous crosslinked polymer blocks which have a very low density and a high capacity of absorbing and retaining liquids. Such high absorption capacity, low density, porous polymer blocks can be prepared by polymerizing a specific type of water-in-oil emulsion known as high internal phase emulsion (HIPE) having relatively small amounts of a continuous oil phase and relatively greater amounts of an internal water phase.
Such high absorption capacity, low density foams are prepared in U.S. Pat. No. 4,522,953 by polymerizing the monomers in the oil phase of a high internal phase water-in-oil emulsion with a water-soluble polymerization initiator such as potassium persulfate. It has been found that in order to obtain foams by this method with high absorption properties and low unreacted monomer content, curing must be conducted for 16 hours or longer at a temperature of 60.degree. C. However, it is desirable to reduce the curing time for a large scale process or a continuous process. Therefore, it will be advantageous to reduce the curing time without significantly affecting the resulting foam properties.
Further, it is desirable to incorporate volatile monomers such as butadiene and isoprene as mentioned in U.S. Pat. No. 5,149,720. However, it is difficult to incorporate these volatile monomers in a low density crosslinked polymeric material ("foam") produced by the high internal phase water-in-oil emulsion. The monomers are difficult to incorporate and tend to volatilize during the preparation and cure of the water-in-oil emulsion. Thus, extremely poor foams in small quantity or closed-cell foams containing these volatile monomers are formed by the conventional high internal phase water-in-oil emulsion method.
It is therefore an object of the present invention to provide a high internal phase water-in-oil emulsion polymerization process with short curing times. It is another object of the present invention to provide an improved method to incorporate volatile monomers in a low density crosslinked polymeric material produced by the high internal phase water-in-oil emulsion process.